Layered adsorption zone for hydrogen production swing adsorption

ABSTRACT

The present invention describes a pressure swing adsorption (PSA) apparatus and process for the production of purified hydrogen from a feed gas stream containing heavy hydrocarbons (i.e., hydrocarbons having at least six carbons). The apparatus comprises at least one bed containing at least three layers. The layered adsorption zone contains a feed end with a low surface area adsorbent (20 to 400 m 2 /g) which comprises 2 to 20% of the total bed length followed by a layer of an intermediate surface area adsorbent (425 to 800 m 2 /g) which comprises 25 to 40% of the total bed length and a final layer of high surface area adsorbent (825 to 2000 m 2 /g) which comprises 40 to 78% of the total bed length.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to adsorption processes and moreparticularly to a pressure swing adsorption (PSA) for separating heavyhydrocarbons from mixtures comprising hydrogen and heavy hydrocarbons.

[0004] The need for hydrogen is increasing for petroleum refiners. Thehydrogen is needed for both the reformulation of gasoline as well as forhydrosulfurization.

[0005] While refiners do have a supply of hydrogen typically from steammethane reformers, they also have hydrogen-rich cracked gas streams fromvarious unit operations, including catalytic cracking and reforming.Typically, these streams are burned for their fuel value. The presenceof hydrogen in the fuel increases NO_(x) formation and the low BTU valueof the fuel decreases crude unit production due to burner limitations.As refiners strive to squeeze more hydrogen out of their plants, theyhave turned their attention to these cracked gas streams. Subsequently,there is considerable interest in recovering essentially pure hydrogenfrom refinery cracked gas streams. Typical feed compositions are 20%methane, 10% ethane, 5% propane, 2% butane, 0.5% pentanes and higherwith the remainder hydrogen. Thus, the desire in the industry is todevelop an adsorption system capable of producing high purity hydrogenfrom hydrogen-rich cracked gas streams which contain heavy hydrocarbons.

[0006] The production and recovery of hydrogen by steam and/or airreforming of hydrocarbon rich gas streams such as natural gas, naphtha,or other mixtures of low molecular weight hydrocarbons is well known inthe art. In PSA processes, a multicomponent gas is passed to at leastone of a plurality of adsorption beds at an elevated pressure to adsorbat least one strongly adsorbed component while at least one componentpasses through. In the case of H₂PSA, hydrogen is the weakly adsorbedcomponent which passes through the bed. At a defined time, the feed stepis discontinued and the adsorption bed is depressurized in one or moreconcurrent steps which permit essentially pure H₂ product to exit thebed. Then a countercurrent desorption step is carried out, followed bycountercurrent purge and repressurization. Such H₂PSA processing isdisclosed by, e.g., U.S. Pat. No. 3,430,418 (Wagner), U.S. Pat. No.3,564,816 (Batta) and U.S. Pat. No. 3,986,849 (Fuderer et al.).

[0007] The production of high purity hydrogen from cracked gas streamscontaining heavy hydrocarbons requires removal of the second most weaklyadsorbing feed gas component, methane, from hydrogen, which is the mostweakly adsorbed component.

[0008] The separation of methane from hydrogen requires a microporousadsorbent, like activated carbon or zeolites. The microporosity isrequired for good selectivity for methane over hydrogen. However,microporous adsorbents, like activated carbons, adsorb C₄—plushydrocarbons very strongly which cannot be desorbed under typical PSAconditions.

[0009] A number of developments relate to PSA processes for removingmethane from hydrogen-containing streams which have significantquantities of C₆₊ (i.e., C_(n) where n≧6) hydrocarbons. For example,U.S. Pat. No. 4,547,205 (Stacey), describes a process for the recoveryof hydrogen and C₆₊ hydrocarbons from a hydrocarbon conversion process.The separation is achieved by first partially condensing out the heavyhydrocarbons. The remaining vapor is then compressed and cooled tofurther condense out heavy hydrocarbons. The pressurized uncondensedcompounds are then sent to a PSA for the production of pure hydrogen.

[0010] In U.S. Pat. No. 5,012,037 (Doshi et al.), an integrated thermalswing-pressure swing adsorption process for hydrogen and hydrocarbonrecovery is disclosed. In this process, a thermal swing adsorptionsystem is used to adsorb heavy hydrocarbons from the feed stream and apressure swing adsorption system is used to remove the remaining lighthydrocarbons to produce a pure hydrogen stream. Of particular interestin both the U.S. Pat. Nos. 4,547,205 and 5,012,037 patents is that C₆₊hydrocarbons are removed prior to PSA.

[0011] Other patents which disclose processes for the recovery ofhydrogen and hydrocarbons from hydrocarbon conversion processes includeU.S. Pat. No. 3,431,195 (Storch et al.) and U.S. Pat. No. 5,178,751(Pappas). Both of these patents disclose processes in whichrefrigeration and partial condensation of heavy hydrocarbons is carriedout prior to introduction to the PSA system.

[0012] U.S. Pat. No. 5,250,088 (Yamaguchi et al.) teaches the use of alayered bed PSA to produce pure hydrogen from a cracked gas stream. Thisinvention teaches a two-layered bed (silica gel followed by activatedcarbon) approach to produce pure H₂, in which the heaviest feed gascomponent is C₅H₁₂. More recently, a two-layer bed approach very similarto that of the U.S. Pat. No. 5,250,088 patent has been published for afeed gas containing C₄H₁₀ (Malek, et al., AlChE Journal, Vol. 44, No. 9,1985-1992 (1998)). In both these cases, the percentage of bed containingsilica gel is about 25%.

[0013] Typically, integrated processes involving thermal swingadsorption (TSA) and/or refrigeration have been utilized to remove thehydrocarbons before introduction to the PSA system. Accordingly, in viewof the above-described need to separate heavy hydrocarbons from amixture comprising hydrogen and heavy hydrocarbons, it is desired toprovide processes which avoid the need to utilize thermal swingadsorption and/or refrigeration prior to PSA to accomplish the desiredseparation.

BRIEF SUMMARY OF THE INVENTION

[0014] This invention provides an improved pressure swing adsorption(PSA) apparatus used to separate heavy hydrocarbons from mixturescomprising hydrogen and heavy hydrocarbons. The apparatus of the presentinvention comprises at least one bed containing at least three layerscomprising a feed-end layer containing a feed-end adsorbent having afirst surface area sufficiently small to separate a heavy hydrocarbonhaving at least six carbons from a light hydrocarbon having less thansix carbons, wherein the first surface area is too small tosubstantially separate methane from hydrogen. The apparatus furthercomprises a product-end layer containing a product-end adsorbent havinga second surface area sufficiently large to separate methane fromhydrogen, and an intermediate layer containing an intermediate adsorbenthaving an intermediate surface area intermediate to said first surfacearea and said second surface area. The invention also provides animproved PSA process utilizing the PSA apparatus of the presentinvention.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0015] The invention will be described in conjunction with the followingdrawings in which like reference numerals designate like elements andwherein:

[0016]FIGS. 1, 2 and 3 are graphs of change in H₂ recovery versuspercentage of bed containing low surface area adsorbent.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The current invention defines optimal adsorbent layering for theproduction of hydrogen from cracked gas streams containing at least sixcarbons (C₆₊ hydrocarbons).

[0018] In preferred embodiments, a PSA apparatus is provided whichcomprises at least one bed containing at least three layers. Theadsorbent layers include a feed-end layer, an intermediate layer and aproduct-end layer. The layers of the present invention are furtherdefined by their 1) surface area and 2) percentage of total bed length.

[0019] The feed-end layer contains a feed-end adsorbent having a firstsurface area sufficiently small to separate a heavy hydrocarbon (i.e., ahydrocarbon having at least six carbons) from a light hydrocarbon (i.e.,a hydrocarbon having less than six carbons), wherein the first surfacearea is too small to substantially separate methane from hydrogen. Withrespect to the feed-end layer, “substantially separate” means that thesurface area is not large enough to produce high purity H₂ (less than100 ppm CH₄) from a gaseous mixture containing methane and hydrogen. Thefeed end of the bed contains adsorbents with a low surface area fromabout 20 to about 400 m²/g, which comprise about 2% to about 20% of thetotal bed length. The feed-end adsorbent has a particle size diameter of0.5 to 3 mm. Preferred feed-end adsorbents are activated alumina, silicagel, titania, silica-alumina or zinc oxide.

[0020] The bed further comprises a product-end layer containing aproduct-end adsorbent having a second surface area sufficiently large toseparate methane from hydrogen, and an intermediate layer containing anintermediate adsorbent having an intermediate surface area intermediateto said first surface area of the feed-end and said second surface areaof the product-end layers. The intermediate layer contains an adsorbentwith an intermediate surface area (from about 425 to about 800 m²/g) andthe product-end layer contains a high surface area adsorbent with a highsurface area (from about 825 to about 2000 m²/g). The intermediate layeradsorbent comprises about 25% to about 40% of the total bed length andthe high surface area adsorbent of the product-end layer comprises about40% to about 73% of the bed. The product end adsorbent has a particlesize diameter of 1 to 3 mm. Preferred intermediate adsorbents are silicagel or activated carbon, and the preferred product-end adsorbents areactivated carbon or zeolite. The intermediate layer adsorbent has aparticle size diameter of 1 to 3 mm. This adsorbent layering allows theintroduction of C₆₊ hydrocarbons into the PSA without the need for otherpretreatment processes such as TSA or refrigeration.

[0021] In another aspect of the invention, a PSA process is employed toprovide purified hydrogen mixtures. The process of the invention employsa PSA apparatus of the invention. The PSA process comprises a highpressure adsorption step comprising introducing a feed gas mixturecontaining hydrogen and heavy hydrocarbons having at least six carbonsinto the feed-end of the adsorbent bed at a high adsorption pressure.The less readily adsorbable component(s) passes through the bed and isdischarged from the product end where a product gas containing at least95% hydrogen is recovered.

[0022] The process is effective for separating hydrogen from gasmixtures containing heavy hydrocarbons and methane. For example, incertain embodiments, a feed gas mixture containing from about 30% toabout 95% hydrogen, and from about 0.005% to about 2% of heavyhydrocarbons can be processed to provide a product gas containing lessthan 1 ppm of said heavy hydrocarbons and at least 99% hydrogen. A feedgas mixture containing at least 2% methane can yield a product gascontaining less than 100 ppm of methane.

[0023] In the adsorption zone, the more readily adsorbable componentsare adsorbed at an adsorption pressure and temperature and the lessreadily adsorbable components are passed through the adsorption zone.Preferred adsorption zone pressures range from about 150 to about 500psig. The adsorption zone temperature is any temperature effective toadsorb the more readily adsorbable components in the feedstream, andpreferably from about 0° C. to about 50° C. (about 32° F. to about 122°F.).

[0024] The invention will be illustrated in more detail with referenceto the following Examples, but it should be understood that the presentinvention is not deemed to be limited thereto.

EXAMPLE 1: COMPARATIVE EXAMPLE

[0025] The feed gas composition for a PSA simulation was 58.4% H₂, 16.8%CH₄, 12.9% C₂H₆, 8.4% C₃H₈, 2.6% C₄H₁₀, 0.6% N₂ and 0.3% C₅H₁₂. The feedpressure was 250 psig at 77° F. (25° C.). A four-bed PSA cycle with twopressure equalizations was simulated. The total bed length of 20.3 feetwas filled with (starting from the feed end of the bed) a low surfacearea adsorbent (activated alumina, 320 m²/g), followed by anintermediate surface area adsorbent (silica gel, 750 m²/g) and a highsurface area adsorbent (activated carbon, 1200 m²/g). The simulation wasused to determine the optimal amount of low surface area adsorbent whilekeeping the high surface area adsorbent amount constant. Thus, thesimulation evaluated the effect of increasing the amount of low surfacearea adsorbent at the expense of the intermediate surface areaadsorbent. The final H₂ product contained 6000 ppm N₂ and 20 ppm CH₄.

[0026]FIG. 1 shows the effect of the addition of a low surface arealayer, i.e. activated alumina, in the PSA bed on the hydrogen recovery.The base bed loading was 6.6 feet (32.5% of the bed) of silica gel and13.7 feet (67.5% of the bed) activated carbon. Introduction of 1.2 feet(5.9% of the bed) of activated alumina, while keeping the amount ofactivated carbon the same, reduced the hydrogen recovery by 0.05percentage points. Increasing the alumina section to 6.6 feet (32.5% ofthe bed) and removing all the silica gel reduced the hydrogen recovery1.4 percentage points. The results from FIG. 1 show that if the heaviesthydrocarbon present in the feed stream is C₅H₁₂, then a low surface arealayer is not needed to optimize the performance of the PSA.

EXAMPLE 2

[0027] Another PSA simulation was performed to determine the performance(hydrogen recovery) of a PSA system for the production of hydrogen andto evaluate the effect of increasing the amount of low surface areaadsorbent at the expense of the intermediate surface area adsorbent. Thefeed gas composition in this example was 55.9% H₂, 16.8% CH₄, 12.9%C₂H₆, 8.4% C₃H₈, 2.6% C₄H₁₀, 0.6% N₂, 2.3% C₅H₁₂ and 0.5% C₆H₁₄. Thefeed pressure was 250 psig at 77° F. (25° C.). As in the example above,a four-bed PSA cycle with two pressure equalizations was simulated. Thetotal bed length of 20.3 feet was filled with a low surface areaadsorbent (activated alumina, 320 m²/g), followed by an intermediatesurface area adsorbent (silica gel, 750 m²/g) and a high surface areaadsorbent (activated carbon, 1200 m²/g). The final H₂ product contained6000 ppm N₂ and 20 ppm CH₄.

[0028]FIG. 2 shows the effect of the addition of alumina to the PSA bedon the hydrogen recovery. As a result, the addition of 1.2 feet ofalumina (5.9% of bed) increased the hydrogen recovery by 0.2 percentagepoints. Increasing the layer of alumina to 3.0 feet (14.8% of bed)improved the H₂ recovery slightly. As the amount of alumina wasincreased to 6.6 feet (32.5% of the bed), the hydrogen recoverydecreased about 1.0 percentage point versus a case where no alumina ispresent. These results indicate that if C₆H₁₂ is present in the feedstream, then the addition of a low surface area adsorbent layer at thefeed end of the PSA bed improves the PSA performance. The percentage ofthe total bed length which contains the low surface area adsorbentshould range from 2 to 20% of the total bed length for optimalperformance.

EXAMPLE 3

[0029] A computer simulation was performed to determine the optimalpercentage of bed length for the intermediate surface area layer usingthe feed and process conditions as in Example 2. In this simulation, thepercentage of the bed containing low surface area adsorbent was heldconstant as the amount of intermediate and high surface area adsorbentswere varied. The results of the simulation are given in FIG. 3. As thepercentage of bed containing intermediate surface area adsorbent (silicagel) increases from 22% of the total bed, the H₂ recovery increases andreaches a maximum at 36% of the total bed length. As the amount of theintermediate surface area increases beyond 36% of the total bed length,the H₂ recovery starts to decrease. The optimal bed loading ofintermediate surface area layer is from 25 to 40% of the total bedlength. The high surface area layer should therefore constitute from 40to 73% of the total bed length.

EXAMPLE 4

[0030] A final computer simulation was performed by to determine theeffect of the particle size of the low surface area adsorbent on PSArecovery. Using the same feed and process conditions as in Examples 2-3,a bed of 5.9% activated alumina, 26.6% silica gel and 67.5% activatedcarbon was simulated with varying particle size of the activatedalumina. When the activated alumina particle diameter was reduced from1.8 mm to 1.0 mm, the H₂ recovery increased 0.1 percentage points. Thisresult shows that a smaller particle adsorbent on the feed end of thebed can improve the overall performance of the PSA system.

[0031] The approach of the current invention is to allow heavyhydrocarbons into the PSA. This can be accomplished by using low surfacearea, large pore adsorbents at the feed end of the PSA bed whicheffectively desorb the heavy hydrocarbons in the process. Since the lowsurface area adsorbents do not have the CH₄ capacity or selectivityneeded to produce high purity hydrogen, the present invention alsoutilizes a three layer bed to produce pure H₂ in which the heaviest feedgas component comprises C₆ and greater hydrocarbons.

[0032] While the invention has been described in detail and withreference to specific examples thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

1. A pressure swing adsorption apparatus comprising at least one bedcontaining at least three layers: a feed-end layer containing a feed-endadsorbent having a first surface area sufficiently small to separate aheavy hydrocarbon having at least six carbons from a light hydrocarbonhaving less than six carbons, wherein said first surface area is toosmall to substantially separate methane from hydrogen; a product-endlayer containing a product-end adsorbent having a second surface areasufficiently large to separate methane from hydrogen; and anintermediate layer containing an intermediate adsorbent having anintermediate surface area intermediate to said first surface area andsaid second surface area.
 2. The apparatus of claim 1, wherein saidfirst surface area is from about 20 m²/g to about 400 m²/g.
 3. Theapparatus of claim 1, wherein said intermediate surface area is fromabout 425 m²/g to about 800 m²/g.
 4. The apparatus of claim 1, whereinsaid second surface area is from about 825 m²/g to about 2000 m²/g. 5.The apparatus of claim 1, wherein said first surface area is from 20m²/g to 400 m²/g, said intermediate surface area is from 425 m²/g to 800m²/g, and said second surface area is from 825 m²/g to 2000 m²/g.
 6. Theapparatus of claim 1, wherein said feed-end adsorbent occupies about 2%to about 20% of a total length of said at least one bed.
 7. Theapparatus of claim 1, wherein said intermediate adsorbent occupies about25% to about 40% of a total length of said at least one bed.
 8. Theapparatus of claim 1, wherein said product-end adsorbent occupies about40% to about 73% of a total length of said at least one bed.
 9. Theapparatus of claim 1, wherein said feed-end adsorbent occupies 2% to 20%of a total length of said at least one bed, said intermediate adsorbentoccupies 25% to 40% of said total length, and said product-end adsorbentoccupies 40% to 73% of said total length.
 10. The apparatus of claim 9,wherein said first surface area is from 20 m²/g to 400 m²/g, saidintermediate surface area is from 425 m²/g to 800 m²/g, and said secondsurface area is from 825 m²/g to 2000 m²/g.
 11. The apparatus of claim10, wherein said feed-end adsorbent is activated alumina, silica gel,titania, silica-alumina or zinc oxide, said intermediate adsorbent issilica gel or activated carbon, and said product-end adsorbent isactivated carbon or zeolite.
 12. The apparatus of claim 11, wherein saidfeed-end adsorbent has a particle size diameter of 0.5 to 3 mm, saidintermediate adsorbent has a particle size diameter of 1 to 3 mm, andsaid product-end adsorbent has a particle size diameter of 1 to 3 mm.13. A process for providing purified hydrogen, said process comprising:providing an apparatus according to claim 1; feeding into a feed end ofsaid apparatus a feed gas mixture containing hydrogen and heavyhydrocarbons having at least six carbons; and recovering from a productend of said apparatus a product gas containing at least 95% hydrogen.14. The process of claim 13, wherein said feed gas mixture contains 30to 95% hydrogen and about 0.005 to about 2% of said heavy hydrocarbons,and said product gas contains less than 1 ppm of said heavyhydrocarbons.
 15. The process of claim 14, wherein said product gascontains at least 99% hydrogen.
 16. The process of claim 14, whereinsaid feed gas mixture further contains at least 2% methane and saidproduct gas contains less than 100 ppm of methane.
 17. The process ofclaim 13, wherein said feed gas is fed into said feed end of saidapparatus at a temperature of 0 to 50° C.
 18. The process of claim 13,wherein said feed gas is fed into said feed end of said apparatus at apressure of 150 to 500 psig.
 19. The process of claim 18, wherein saidfeed gas is fed into said feed end of said apparatus at a temperature of0 to 50° C.
 20. The process of claim 13, wherein said first surface areais from about 20 m²/g to about 400 m²/g.
 21. The process of claim 13,wherein said intermediate surface area is from about 425 m²/g to about800 m²/g.
 22. The process of claim 13, wherein said second surface areais from about 825 m²/g to about 2000 m²/g.
 23. The process of claim 13,wherein said first surface area is from 20 m²/g to 400 m²/g, saidintermediate surface area is from 425 m²/g to 800 m²/g, and said secondsurface area is from 825 m²/g to 2000 m²/g.
 24. The process of claim 13,wherein said feed-end adsorbent occupies about 2% to about 20% of atotal length of said at least one bed.
 25. The process of claim 13,wherein said intermediate adsorbent occupies about 25% to about 40% of atotal length of said at least one bed.
 26. The process of claim 13,wherein said product-end adsorbent occupies about 40% to about 73% of atotal length of said at least one bed.
 27. The process of claim 13,wherein said feed-end adsorbent occupies 2% to 20% of a total length ofsaid at least one bed, said intermediate adsorbent occupies 25% to 40%of said total length, and said product-end adsorbent occupies 40% to 73%of said total length.
 28. The process of claim 27, wherein said firstsurface area is from 20 m²/g to 400 m²/g, said intermediate surface areais from 425 m²/g to 800 m²/g, and said second surface area is from 825m²/g to 2000 m²/g.
 29. The process of claim 28, wherein said feed-endadsorbent is activated alumina, silica gel, titania, silica-alumina orzinc oxide, said intermediate adsorbent is silica gel or activatedcarbon, and said product-end adsorbent is activated carbon or zeolite.30. The process of claim 29, wherein said feed-end adsorbent has aparticle size diameter of 0.5 to 3 mm, said intermediate adsorbent has aparticle size diameter of 1 to 3 mm, and said product-end adsorbent hasa particle size diameter of 1 to 3 mm.